Process for increasing the adhesion of a metal surface to a polymer

ABSTRACT

A process for increasing the adhesion between a copper or copper alloy surface and a polymeric material is disclosed. The process comprises treating the copper or copper alloy surface with an aqueous solution of cupric ions at a pH from 2.8 to 4.2 to form an oxide conversion coating on the copper or copper alloy surface. The so treated surface can then be bonded to the polymeric material.

BACKGROUND OF THE INVENTION

The present invention relates to a composition and process for improving the adhesion between a metal and polymer, especially between copper foil and a polymeric substrate in the fabrication of printed circuit boards. The composition and process are also useful in increasing the adhesion between lead frames or chip carriers and encapsulating resins.

Printed circuit boards are typically constructed with a polymeric substrate such as a phenolic, epoxy, polyimide, polyester, or other resin, upon which is bonded a copper foil which is etched to provide a conductor in the desired configuration. Copper, like other pure metals, generally exhibits poor adhesion characteristics for bonding to polymers, and intermediate conversion coatings are frequently helpful. Familiar examples of this practice are the various pre-paint treatments, such as phosphate on steel, chromate on zinc or aluminum, and anodic oxide on aluminum.

For printed circuits, it is typical to employ copper oxide as adhesion improving coating, however, the known methods for achieving the copper oxide coatings involve the use of highly corrosive chemicals which become even more dangerous at the elevated temperatures typically employed for processing. Moreover, the improvement in adhesion achieved through the use of known technology has been erratic. In fact, adhesion after treatment is sometimes no better than for clean copper.

In view of these difficulties, the question arises as to why use copper oxide at all when clean etched copper alone will give peel strengths of two to three pounds per inch. However, it is known that copper is a reactive metal and can react with the components or decomposition products of the polymer substrate. For instance, copper can compete for the amine curing agents in epoxy resins. These chemical and physical processes, aggravated by thermal cycling, can continue for the life of the assembly and lead to ultimate failure in service. An oxide coating serves as a diffusion barrier to prevent these reactions. Accordingly, there is a present need for improvements in achieving increased bonding strength through the use of copper oxide coatings.

In preparation of printed circuits, a copper foil is bonded to the polymeric substrate which may be phenolic, epoxy, polyimide, polyester, or the like. Prior to bonding, the foil is normally treated electrolytically to provide a specific surface structure as disclosed in U.S. Pat. Nos. 3,292,109; 3,318,758; 3,518,168; 4,049,481; 4,131,517; 4,176,035; and others. It is believed that the copper foil on most single and two-sided printed circuit boards is so treated.

Multilayer printed circuit boards are assemblies of several two-sided boards further bonded to each other through layers of semicured polymeric material which are subsequently cured at elevated temperatures and pressures to form the complete assembly. Prior to this assembly, the copper foil of the two-sided boards is imaged and etched to from the inner layer circuits. The opposite side of the conductor patterns must then be treated for adhesion to the layer of polymer bonding one board to another. Because the etched conductors have no continuity for electrolysis, it is necessary to treat them chemically.

Copper oxide is the most useful chemical conversion coating for copper adhesion and has been used since the early days of printed circuit technology. The basic patent in this field appears to be U.S. Pat. No. 2,364,993, which disclosed the use of sodium chlorite and sodium hydroxide at high concentrations and temperatures near boiling. Similar disclosures appear in other patents, such as U.S. Pat. Nos. 2,460,896, 2,460,898, and U.S. Pat. No. 2,481,854, which have been granted to the same assignee. These four related patents teach the use of a treatment solution containing caustic in an amount which equals or exceeds the chlorite. The concentration range disclosed is from five grams per liter of chlorite and ten grams per liter of caustic on the low side, to solutions saturated with chlorite and containing one thousand grams per liter of caustic. It is indicated that the lower range will blacken copper in thirty minutes at 216 degree F., while the higher range will do the same in one minute at 250 degree F. An intermediate composition containing 150 grams per liter of chlorite and 150 grams per liter of caustic is disclosed in U.S. Pat. No. 2,364,993 to blacken the copper surface in five minutes at 200 degree F.

While these patents appear to be directed toward decorative applications, as well as non-reflective coatings for the interior surfaces of optical instruments, the same principles have been applied to the treatment of copper foils for use in preparing printed circuits. Some exemplary patents applying this technology to printed circuit adhesion are: U.S. Pat. Nos. 2,955,974; 3,177,103; 3,198,672; 3,240,662; 3,374,129; and U.S. Pat. No. 3,481,777. Other known means for providing copper oxide coatings include oxidation with alkaline permanganate as disclosed in U.S. Pat. No. 3,544,389, hydrogen peroxide as disclosed in U.S. Pat. No. 3,434,889, and even air at elevated temperature as disclosed in U.S. Pat. No. 3,677,828.

It is known that these oxide coatings are erratic and too frequently provide surprisingly poor adhesion, sometimes less than clean copper. A part of this problem may be that the oxide coatings are often too thick-presumably because it was thought that thicker coatings were better because of the increased specific surface area. In actual fact, a thick oxide is inferior as an adhesion promoter because it is mechanically weak and may not be homogeneous.

A thick coating of oxide tends to be fuzzy and velvety, making it mechanically weak. Black powder can be wiped off with a finger. During the multilayer processing operation at a high temperature and pressure, the fibers can be crushed and partially encapsulated by the flowing polymer. This means that the bond can break within the oxide layer, causing a cohesive, rather than adhesive, failure. The so-called “oxide transfer”, seen as a dark stain occasionally appearing within the epoxy surface after etching away the copper, is a manifestation of encapsulated oxide fibers.

Further, a thick black oxide may not be homogeneous. Whereas the outer surface will be cupric oxide, there will be a gradient through the thickness which will be increasingly richer in cuprous oxide. This is an unstable species which can be oxidized to cupric oxide during the high temperature pressing operation by combining with residual oxygen, water or various decomposition products. The result of this reaction is a change in volume, a breaking away of points of attachment, and the creation of micro-voids which lead to poor adhesion.

In view of these difficulties, it would be desirable to provide a thermally-stable oxide coating on copper to enhance its adhesion characteristics to polymers. Additionally, it would be desirable to provide a solution capable of forming such an oxide coating which was sprayable in conventional spray equipment, such as that typically employed for spray etching printed circuit boards to enable increased productivity in continuous, conveyorized applications. And, it would be desirable to provide a composition and a process for forming oxide coatings on copper which could be operated at concentrations and temperatures below about 140 degree F.

SUMMARY OF THE INVENTION

Thus a new process is proposed for creating an oxide coating on the surface of copper or a copper alloy, said process comprising:

(a) contacting the copper or copper alloy surface with a composition comprising:

-   -   1. water; and     -   2. a source of cupric ions;         -   at a pH from about 2.8 to 4.2;

The copper or copper alloy surface can then be effectively bonded to polymeric substrates or coatings with greater adhesion and stability.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a unique composition and process for the formation of an oxide conversion coating on a copper or copper alloy surface which provides for improved adhesion between metal and polymeric resins as in the manufacture of multilayer printed circuit boards or lead frame components. The proposed process comprises:

(a) contacting the surface of copper or copper alloy with a composition comprising:

-   -   1. water; and     -   2. a source of cupric ions;         -   at a pH from 2.8 to 4.2;             such that a conversion coating is formed on the copper or             copper alloy surface. Preferably, the composition of this             invention does not contain any azole compounds or other             corrosion inhibitors. The composition is also preferably             substantially free of hydrogen peroxide. The conversion             coating formed is resistant to scratching and to             discoloration.

The source of cupric ions can be any soluble source of cupric ions such as cupric salts like cupric sulfate, cupric chloride, cupric acetate and mixtures of the foregoing.

The pH of the composition is important. The pH should be between 2.8 and 4.2, preferably between 3.0 and 4.0 and most preferably between 3.2 and 3.8. Adjustment of the pH can be made with any acid or base but is preferably made with an acid corresponding to the anions of the cupric source, if acid is used, or sodium hydroxide if base is used. It is especially surprising and unexpected that the copper oxide conversion coating forms in a stable way at an acidic pH.

Since pH is important, it is preferable for the composition to also comprise a buffer system to control and maintain the pH within the recommended range. Any buffer system that buffers the solution within the prescribed pH range is acceptable. The inventor has found the following buffer systems to be useful in this regard: sodium or potassium carbonate, sodium citrate and sodium hydrogen phosphate, and/or potassium hydrogen phthalate and sodium hydroxide.

The copper or copper alloy surfaces can be contacted with the composition of this invention by immersion, spray or flood. The temperature of the composition should be between 120° F. and 150° F., preferably between 130° F. and 140° F. The contact time varies with the method of contact and the temperature, but can range from 15 seconds to 5 minutes.

Before the copper or copper alloy is treated with the composition of this invention, the surface is preferably cleaned and microetched. The cleaner can be any standard commercial acid or alkaline cleaner which is designed to clean copper surfaces. Suitable commercial cleaners include Core Clean AT and Omniclean CI available from MacDermid, Incorporated. After the surface is cleaned, it is also preferably microetched to roughen and activate the surface. Suitable micro etches include peroxide/sulfuric or cupric based microetches such as Microetch G-5, Microetch G-4 and Multiprep 100 available from MacDermid, Incorporated. The important point is for the surface to be clean and microroughened before entering the composition of this invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an electron microscope photograph showing the surface of an oxide coating prepared by the process of this invention.

The invention is further described in a non-limiting manner in the following example:

EXAMPLE 1

Copper foil was processed as follows:

-   -   1. Clean in Omniclean CI alkaline cleaner at 140° F. for 5         minutes.     -   2. Rinse.     -   3. Microetch in Multiprep 100 microetchant at 100° F. for 1.0         minutes.     -   4. Rinse.     -   5. Immerse in the following composition of this invention at         140° F. for 1.5 minutes:         -   68 g/l copper sulfate pentahydrate         -   5 g/l potassium carbonate         -   pH=3.85     -   6. Rinse.     -   7. Dry.

The copper foil was then laminated to an epoxy prepreg using heat and pressure and its adhesion properties were checked with the following results:

-   -   Initial Adhesion −7.0 lb/in.     -   Adhesion after 3-10 sec. solder shocks at 550° F.-6.5 lb/in.     -   Adhesion after 6-10 sec. solder shocks at 550° F.-4.5 lb/in. 

1. A process for increasing the adhesion of a copper or copper alloy surface to a material comprising polymers, said process comprising: a) contacting the copper or copper alloy surface with a composition comprising:
 1. water;
 2. a source of cupric ions; at a pH from 2.8 to 4.2; such that a conversion coating is formed on the copper or copper alloy surface.
 2. A process according to claim 1, wherein the composition is substantially free of azole compounds.
 3. A process according to claim 1 wherein the composition also comprises a buffer.
 4. A process according to claim 4 wherein the buffer is selected from the group consisting of potassium or sodium carbonate, sodium citrate and sodium hydrogen phosphate, and potassium hydrogen phthalate and sodium hydroxide.
 5. A process according to claim 1 wherein the source of cupric ions is selected from the group consisting of cupric sulfate, cupric chloride, cupric acetate and mixtures of the foregoing.
 6. A process according to claim 1 wherein the pH is from 3 to
 4. 7. A process according to claim 3, wherein the composition is substantially free of azole compounds.
 8. A process according to claim 7 wherein the buffer is selected from the group consisting of potassium or sodium carbonate, sodium citrate and sodium hydrogen phosphate, and potassium hydrogen phthalate and sodium hydroxide.
 9. A process according to claim 8 wherein the source of cupric ion source selected from the group consisting of cupric sulfate, cupric chloride, cupric acetate, and mixtures of the foregoing.
 10. A process according to claim 9 wherein the pH is from 3 to
 4. 